Linear Resonant Actuators (LRAs) are devices which may be used to stimulate the vibro-tactile sensing system of the human body in order to elicit touch sensations programmatically. The Pacini neuron in the human tactile system is particularly sensitive to vibrations of a frequency within the range 100 Hz to 400 Hz. LRAs may be used to stimulate the tactile system directly through controlled vibrations. These vibrations may be achieved by applying an electromechanical force to a small mass held by a spring, or set of springs. The electromechanical force may be elicited by applying an input voltage (usually oscillatory) to the LRA which makes the inner mass of the LRA move.
FIG. 1 illustrates an example of a haptic transducer 100. The moving mass 102 is centred in a rest position by a pair of springs 104a and 104b. The moving mass 102 comprises one or more permanent magnets 106a sand 106b embedded within it, and one or more coils of wire 108 may apply electromagnetic force to the magnets, thereby moving the moving mass 102 from the rest position, usually in an oscillatory manner. It will be appreciated that FIG. 1 illustrates a basic configuration of a haptic transducer 100, and multiple-magnet and/or multiple-coil configurations are all available. The current applied to the coil 108 moves the moving mass 102 with respect to a housing of the haptic transducer 100. The moving mass 102 may then vibrate within the housing, and stops 110a and 110b limit the excursion of the moving mass 102 from the rest position. The stops 110a and 110b may therefore limit spring damage if the driving force is too high.
FIG. 2 illustrates an example of a control system 200 for controlling the driving signal applied to a haptic transducer 201. The voltage and current across the terminals of the haptic transducer may be measured, and a haptic waveform generator 202 may monitor the measured voltage and current signals in order to drive the LRA to a desired motion.
The haptic transducer 201 may have limited available excursion within the housing until it hits the stops. Hitting the stops may generate an unwanted haptic or audible response, and may also cause damage to the haptic transducer 201 especially if repeated several times. There may therefore be a need for controlling the maximum excursion inside a haptic transducer. In other transducers, similar problems, such as for example with micro loudspeaker protection, the excursion may be measured directly by use of a laser. However, particularly for haptic transducers, but potentially in scenarios where the use of a laser is either unsuitable or undesirable for economic reasons or otherwise, it may not be possible to measure the excursion of the transducer directly.
For haptic transducers, it may be possible to open the housing enough to be able to measure the movement of the mass with a laser. However, the process is not only difficult to perform, but even when successful, a change in the system is observed due to the modifications caused by physically opening the casing. Furthermore, it is not a feasible way to approach a distribution of produced haptic transducers as the measurement may have to be performed on a statistical set of the component. A modified component, in which the casing has been opened, cannot usually be mounted in the actual end product, making the measurement by using a laser a difficult way to tune the haptic transducers in the development of a larger product such as a mobile phone.